VLBI study of maser kinematics in high-mass star-forming regions

Abstract

Aims. We performed a detailed study of maser and radio continuum emission toward the high-mass star-forming region G23.01−0.41. This study aims at improving our knowledge of the high-mass star-forming process by comparing the gas kinematics near a newly born young stellar object (YSO), analyzed through high spatial resolution maser data, with the large-scale environment of its native hot molecular core (HMC), identified in previous interferometric observations of thermal continuum and molecular lines. Methods. Using the VLBA and the EVN arrays, we conducted phase-referenced observations of the three most powerful maser species in G23.01−0.41: H 2 O at 22.2 GHz (4 epochs), CH 3 OH at 6.7 GHz (3 epochs), and OH at 1.665 GHz (1 epoch). In addition, we performed high-resolution (≥0. 1), high-sensitivity (<0.1 mJy) VLA observations of the radio continuum emission from the HMC at 1.3 and 3.6 cm. Results. We have detected H 2 O, CH 3 OH, and OH maser emission clustered within 2000 AU from the center of a flattened HMC, oriented SE-NW, from which emerges a massive 12 CO outflow, elongated NE-SW, extended up to the pc-scale. Although the three maser species show a clearly different spatial and velocity distribution and sample distinct environments around the massive YSO, the spatial symmetry and velocity field of each maser specie can be explained in terms of expansion from a common center, which possibly denotes the position of the YSO driving the maser motion. Water masers trace both a fast shock (up to 50 km s −1 ) closer to the YSO, powered by a wide-angle wind, and a slower (20 km s −1 ) bipolar jet, at the base of the large-scale outflow. Because the compact free-free emission is found offset from the putative location of the YSO along a direction consistent with that of the maser jet axis, we interpret the radio continuum in terms of a thermal jet. The velocity field of methanol masers can be explained in terms of a composition of slow (4 km s −1 in amplitude) motions of radial expansion and rotation about an axis approximately parallel to the maser jet. Finally, the distribution of line-of-sight velocities of the hydroxyl masers suggests that they can trace gas less dense (n H 2 ≤ 10 6 cm −3 ) and more distant from the YSO than that traced by the water and methanol masers, which is expanding toward the observer. A few pairs of OH masers, with different circular polarization, are well aligned in position on the sky and we interpret them as Zeeman pairs. From Zeeman splitting, the derived typical values of the magnetic field are of a few mG.

A subarcsecond study of the hot molecular core in G023.01−00.41

“…1: 1) the flattened morphology of the core, perpendicular to the outflow direction; and 2) the velocity field revealed through the CH 3 CN emission, which appears to outline two mutually orthogonal directions on subarcsecond scales ( < ∼ 3 × 10 3 AU). We also recall that the HMC is known to contain a compact source of radio continuum emission and a number of CH 3 OH and H 2 O maser features (Sanna et al 2010, their Fig. 4).…”

“…The correlator was configured to an almost uniform spectral resolution of 0.81 MHz (∼1.05 km s −1 ) over the entire bandwidth, except for the spectral windows covering the strongest CH 3 CN lines (K = 0-6) for which a resolution of 0.41 MHz (0.52 km s −1 ) was used. The phase center of the observations was the putative position of the YSO inferred from masers and radio continuum observations (see Table 1; Sanna et al 2010). Doppler tracking was performed assuming an LSR velocity of 77.4 km s −1 , corresponding to the rest velocity of the CH 3 CN (6 K -5 K ) lines (Furuya et al 2008).…”

“…The 15 4 −16 3 methanol line was selected as the highest excitation (E low ≈ 363 K) and strongest transition visible in the VEX data, with the aim to compare the thermal methanol emission with the 6.7 GHz methanol maser emission studied by Sanna et al (2010). The line profile (Fig.…”

“…The dashed black line is the best linear fit to the peak distribution, which has a slope dv/dx = −22.4 ± 2.4 km s −1 arcsec −1 , and a correlation coefficient of r = 0.8. The red triangles represent the CH 3 OH maser spots detected by Sanna et al (2010). d) Same as panel c) for the cut along the major axis of the elongated HMC (PA = −32 • ); east offsets are measured along this cut.…”

“…G023.01−00.41 is a HMSFR located at a trigonometric distance of 4.59 +0.38 −0.33 kpc (Brunthaler et al 2009), which exceeds a bolometric luminosity of 10 4 L (see Sanna et al 2010, for a recent review). On a parsec scale, Spitzer/GLIMPSE images of the field reveal bright, "green fuzzy" emission at 4.5 μm, believed to be associated with an intense outflow activity (e.g., Cyganowski et al 2009).…”